All the emergency sirens in the US city of Dallas were set off for about 90 minutes in one of the largest known breaches of a warning system.
Dallas' 156 sirens, normally used to warn of tornadoes and other dangerous weather, were triggered at 11.42 pm on Friday US time.
The wailing did not end until 1.17 am when engineers manually shut down the sirens' radio system and repeaters, city emergency management director Rocky Vaz said.
"At this point, we can tell you with a good deal of confidence that this was somebody outside of our system that got in there and activated our sirens," he told reporters.
The breach in the city of 1.6 million people was believed to have originated in the area, city spokeswoman Sana Syed said in a statement.
Vaz cited industry experts as saying the hack was among the largest ever to affect emergency sirens, with most breaches triggering one or two.
"This is a very, very rare event," he said.
Dallas relied on local media, emergency 911 phone calls, and a federal radio alert system until it was able to restore the sirens.
The hack is being investigated by system engineers and the Federal Communications Commission has been contacted, but police have not been involved, Vaz said.
The sirens went through 15 cycles of a 90-second activation before they were shut down, he said.